1. Field of the Invention
The present invention relates to a noise reduction apparatus for removing white noise contained in a composite video signal, incorporating frame memories, for use in a television receiver or video cassette recorder which employs digital processing of luminance and color signals, and in particular to a noise reduction apparatus for operating on an NTSC format composite video signal.
2. Description of Related Art
In recent years, various types of noise reduction apparatus utilizing frame memories have begun to be incorporated into high definition or improved definition types of digital operation television receiver, and in particular into television receivers which use the IDTV (Improved Definition Television) system for the NTSC format. Generally, such a television receiver is provided with a 3-dimensional Y/C separator circuit or a line-sequential scanning conversion circuit, utilizing one or more frame memories. In addition, types of noise reduction circuits have been proposed for such a television receiver which are based on correlation between the video signal in successive frames, using a frame memory as a delay element. The operation of such a noise reduction circuit is basically intended to reduce noise contained in those portions of the video signal which do not correspond to moving image regions, i.e. the portions of the video signal which should ideally be perfectly correlated between successive frames. Such a noise reduction circuit functions by producing a noise-reduced video signal in which amounts of change (e.g. for each pixel) occurring between successive frames are reduced from those which occur in the input video signal. Such a noise reduction circuit will therefore provide incorrect operation if it acts on portions of the input video signal which actually represent moving image regions, and will result in unwanted residual image regions appearing in the finally obtained display picture.
It is therefore desirable to control such a noise reduction circuit to provide a maximum degree of noise reduction when the input video signal corresponds to static image regions, and to provide zero or a very low degree of noise reduction when the video signal represents a moving region. With one prior art noise reduction apparatus, manual control of the degree of noise reduction produced by such a noise reduction circuit is provided, i.e. the user can adjust the degree of noise reduction while observing the resultant displayed picture, to effect a visually satisfactory result. With another prior art method, as described in detail hereinafter, the degree of noise reduction is made to vary automatically by establishing a non-linear relationship between that degree and the degree of correlation between successive frames of the input video signal, such that if there is a very low degree of correlation, it is judged as resulting from motion components in the video signal, so that the noise reduction circuit is made inoperative. However, that method has format disadvantage, in the case of a television format such as format NTSC format in which the chroma subcarrier is phase-inverted in successive frames, that color interference components of static image regions (sometimes called "cross-color" interference, resulting from interaction between the chroma signal and high-frequency components of the luminance signal in a composite video signal) will be incorrectly judged as representing motion components in the input video signal, so that the effectiveness of the noise reduction circuit will be low.